As the US military was looking to expand production of Boeing's B-17 bomber in the late 1930s, Washington asked Consolidated Aircraft to start producing the plane.

But after a visit to Boeing's factory in Seattle, Consolidated proposed a totally new aircraft.

Consolidated was granted a design study for a new bomber with specifications exceeding those of the B-17. The company quickly turned out a new design and received a contract for a prototype, the XB-24, in March 1939.

On December 29, 1939, the XB-24 took its first flight — months after the Nazi Blitzkrieg swept over Poland in September. By spring 1940, Adolf Hitler's forces were marching through Western Europe, and Consolidated's new bomber was sent to the British.

US hadn't entered the war, but President Franklin Roosevelt exhorted US industry to shift from peacetime production to churning out materials needed to bolster Allied forces on the brink of defeat.

President Franklin Roosevelt at the commissioning of four B-24 Liberator bombers and their delivery to US-trained Yugoslavian crews, in Washington, DC, October 6, 1943. (AP Photo)

"Guns, planes, ships, and many other things have to be built in the factories and the arsenals of America. They have to be produced by workers and managers and engineers with the aid of machines which in turn have to be built by hundreds of thousands of workers throughout the land," Roosevelt said on December 29, 1940. "We must be the arsenal of democracy."

The B-24 bomber, dubbed the Liberator, would become a mainstay of that arsenal. The hearty bomber saw service in all theaters of the war but played an essential role in the effort to pummel German forces in Europe.

"The B-24 has guts," the Army Air Force's pilot-instruction manual said. "It can take it and dish it out. It can carry a bigger bomb load farther and faster, day in and day out, than any airplane that has passed the flaming test of combat."

Below, you can see how the vaunted B-24 went from prototype to the most mass-produced aircraft in history — helping carry Allied forces to victory along the way:

By the beginning of 1941, other manufacturers had joined the effort to build B-24s. The Ford Motor Company made the audacious promise to build one bomber every hour — a claim that drew derision from the aircraft industry, which doubted an automobile company was capable of such a feat.

A Consolidated B-24 bomber tests its wings in a flight over the Pacific in San Diego, December 3, 1940, before flying to England. The first 26 of these huge aircraft were earmarked for England. This one already is camouflaged and bears the Royal Air Force insignia. (AP Photo)

In January 1942, when Edsel Ford, the president of the company named after his father, first saw the B-24 at the Consolidated Aircraft factory in San Diego, he said, "It appeared a monstrosity." A prototype had been completed a year earlier, and the finished product was 66 feet and 4 inches long and 17 feet and 11 inches tall, with a wingspan of 110 feet — the longest aircraft of any kind in the US.

A partial view of an assembly line in the Consolidated Aircraft Corporation’s new plant between working shifts on May 21, 1942. (AP Photo)

The wings were "exceptionally long and unusually narrow, with a high-aspect ratio that provided extraordinary lift," A.J. Baime writes in "The Arsenal of Democracy." The wing "was mounted shoulder-level on the fuselage so that it looked like arms outstretched, and the four engines hung down."

The Ford team found it would need 5 miles of wire, cut into almost 3,000 pieces ranging in length from 8 inches to 32 feet, for a B-24. The plane's components were 85% aluminum alloy and 13% steel. The rest was a mix of magnesium, brass, plastic, rubber, and other materials.

Each bomber also required 360,000 rivets — some one-sixteenth of an inch long and weighing .00005 pounds; others 50 times as long and weighing 0.05 pounds.

Ford and his top engineer, Charles Sorensen, believed Consolidated had "created a hell of a weapon" but criticized its production method, which was time-consuming and done in open air, exposing the plane to the heat and cold. The Ford team knew how to build a lot of B-24s quickly: mass production on an assembly line.

In a section of the huge interior of Ford's Willow Run bomber plant, workmen build the center wing of the B-24, July 11, 1942. Sections of the bombers were shipped to other assembly plants while finishing touches to put the plant into all-out production continued. (AP Photo)

Depending on the model, the B-24 was manned by a crew of between seven and 10 men stationed around the plane's cramped confines. The bombardier, stationed in the belly, controlled the payload doors. The flight crew, including the navigator and flight engineer, also manned the bomber's machine-gun turrets, which were set in the nose, tail, spine, and belly.

Twenty seven gauges and 12 levers spread across the instrument panel in the cockpit, used to control the bomber's speed and fuel. The four engines could put out a total of 4,800 horsepower, then the equivalent of 56 Ford V8s.

"To compare a Ford V8 with a four-engine Liberator bomber was like matching a garage with a skyscraper," Sorensen wrote. But he formed a plan to build B-24s that won over Edsel Ford: Make them on an assembly line housed in the largest factory in the world, a mile long and a quarter-mile wide, churning out one heavy bomber an hour. The plan would lead to the construction of the massive Willow Run plant near Ypsilanti.

At Ford’s Willow Run plant, B-24 Liberator bombers come off production lines. Workers put final touches on the basic wing sections, March 3, 1943, while others begin to install the wheel mount, wheel, and tire. (AP Photo)

The government initially rejected the proposal, but Ford secured a $480 million contract for 1,200 "knockdown" B-24 airframe assemblies — everything but the engines delivered in pieces — and 800 complete planes, on March 3, 1941.

The Ford team encountered numerous problems in its push to get production going. New blueprints had to be made; the new plant needed custom-made tools and equipment; the plant had to be staffed, and the staff had to be trained and housed.

Throughout the war, the Ford Company would have navigate labor disputes and racial tensions at its plant and in nearby Detroit. The fraught relationship between Edsel and his father, and their fight for control of the company's future, added to the drama.

The attack on Pearl Harbor solidified the belief among US leaders that air power was essential. Roosevelt's secretary of war, Henry Stimson, told the president that the four-engine bomber had caused a "reversal of the strategy of the world." Roosevelt himself was reportedly "a believer in bombing as the only means of gaining a victory." But reports that Germany was producing advanced, versatile aircraft in fearsome numbers worried the Allies — Roosevelt himself watched US aircraft production numbers intently throughout the war.

Two trucks carrying half-ton bombs move up to “The Eager Beaver,” a US B-24 Liberator bomber in the Southwest Pacific on May 28, 1943, in preparation for a raid against the Japanese. Bombers of the Southwest Pacific command smashed Lae, New Guinea on May 27. (AP Photo/Ed Widdis)

The first piece of B-24 was completed at Willow Run two days after the attack. But Edsel had promised that the massive bombers would take flight in May 1942.

After numerous delays and production problems, Bomber Ship 01 took off from the air field adjacent to the Willow Run plant on May 15, 1942. (Though it would be months before Ford hit its bomber-a-month production goal.)

"It was HUGE. I was completely amazed by its monstrous size, its four mighty engines," a pilot who flew the B-24 said upon first seeing the bomber.

Winston Churchill picked an American-made B-24 to fly him to the meeting of Allied leaders at Casablanca in early 1943. Its bomb racks had been replaced with passenger seating.

By the start of 1942, four companies were contracted to build B-24s: Consolidated Aircraft, which was the original designer, Douglas, North American, and Ford, the only firm that was not originally an aircraft manufacturer.

The B-24 was designed to be the US's fastest heavy bomber. Its top speed was 300 mph, and its 3,000-mile range was more than any other US plane. Its maximum payload, 8,000 pounds, was more than any other US bomber could carry. When it rolled down the runway on its tricycle landing gear — a new design feature for bombers — its takeoff weight could be up to a colossal 60,000 pounds. The front tire, itself 3 feet tall, could carry 27,000 pounds on its own.

Photographer Sgt. John A. Boiteau took this photo of a B-24 Liberator in 1943 while a Liberator during a bombing run over Salamau, New Guinea, before its capture by Allied forces. Bomb bursts can be seen below in lower left and a ship at upper right along the beach. (AP Photo/US Army Force)

The bomber had a thin aluminum skin, and the pilots sat in cast-iron seats to protect them from incoming flak and gunfire. Some 4,000 feet of rubber and metal tubing coursed through the B-24, shunting fuel and fluids through the aircraft.

The B-24 carried 18 rubber fuel cells, 12 in the center wing and three more in both outer wings. The cells carried 16,320 pounds, or 2,720 gallons, of 100-octane gasoline, and if a bullet was shot through them, they were self-sealing. The bomb hold was designed to carry the 8,000 payload three ways: four 2,000-pound bombs, eight 1,000-pound bombs, or 12 500-pound bombs and 20 100-pound bombs.

Each bomber was held together by 360,000 rivets — some one-sixteenth of an inch long and weighing .00005 pounds; others were 50 times as long and weighed 0.05 pounds.

B-24s operated in every theater of the war. Its range allowed it to cover distances between targets in the Pacific and to shield Atlantic shipping lanes from roving U-boats. Before Allied leaders left their first meeting at Casablanca in January 1943, US commanders had an ambitious plan for the Army Air Corps' most important target in Nazi-occupied Europe: Ploesti, a Romanian city home to sprawling oil refineries. A successful strike there could take out one-third of Hitler's oil production. The city was "the taproot of German might," Churchill said.

B-24 Liberator bombers of the US 8th Air Force flying to targets in occupied Europe on August 12, 1943. (AP Photo)

Flying from Benghazi in the Libyan desert, it would be a round trip of roughly 2,400 miles. The B-24 was the only bomber with the range and firepower to reach Ploesti and take out the facilities there. Allied commanders called the mission Operation Tidal Wave and scheduled it for August 1943.

But Ploesti was also the most heavily defended oil installation on the planet, with radar-warning systems and countless batteries of German 88-millimeter anti-aircraft guns, with which Allied flyers in Europe had become all too familiar.

The first Allied effort to knock out the refineries at Ploesti was costly and unsuccessful. Operation Tidal Wave kicked off on August 1, 1943, with 178 B-24s manned by 1,763 Army airmen leaving Libya laden with extra fuel for the 13-hour flight. They carried "more killing power than two Gettysburgs," an Air Corps journalist said. But the armada was detected by German sentries in Greece. The mission was further complicated by dense cloud cover north of Greece. Confused and disorganized, the US crews flew into a dense thicket of Nazi anti-air defenses.

A 9th Air Force B-24 over blazing oil refineries in Ploiesti, Romania. Ninety percent of the damage was caused by delayed-action bombs after these photographs were taken on August 16, 1943. (AP Photo)

The entire attack on Ploesti lasted 27 minutes. The B-24s who survived the onslaught over the city turned for home, pursued by German fighters that picked off more American bombers.

Just 88 B-24s made it back to Benghazi, and 446 American airmen were killed or missing in an attack that only knocked out less than half of Ploesti's refinery operations for a brief period.

"We flew through sheets of flame, and airplanes were everywhere. Some of them on fire and others exploding," said Col. Leon Johnson, commander of the 44th Bomb Group, of the attack on Ploesti. "It's indescribable to anyone who wasn't there."

After an attack on Wewak, a B-24 returned to base in the Southwest Pacific full of bullet holes and with the nose wheel shot away, August 29, 1943. One crew member was killed and several injured. Injured members of the crew are being taken from the plane and put into ambulances. (AP Photo/Edward Widdis)

The Army Air Corps took heavy losses of men and machines, but prodigious US production kept sending aircraft to the front.

During their conference at Casablanca in January 1943, Allied planners agreed on a strategy for their bombers. American bombing efforts would focus on strategic targets — factories, ports, military bases, and other infrastructure vital to Germany's military output — during the daytime.

At night, British bombers targeted German cities, in what many felt was retribution for the Nazi Luftwaffe's savage attacks during the Blitz and the Battle of Britain.

In April 1944, Allied bombers returned to Ploesti. On the first mission alone, nine groups of B-24s and four groups of B-17s dropped almost 1.2 million tons of TNT on the city. The columns of smoke they left behind could be seen tens of miles away. "We really clobbered them that day," said Bill Harvey, a B-24 bombardier. Airstrikes continued over the next few weeks, bookended by a May 31 bombardment carried out by 428 B-24s. The strikes dealt serious blows to the Nazi war machine. German planes were grounded and panzers stopped in their tracks.

B-24 liberators of the US 8th Air Force, operating under the Northwest African Air Forces, bomb Bastia, Corsica, France, on October 2, 1943. Hits were made on a large merchant vessel and three small merchant ships. (AP Photo)

At home, more planes rolled off the assembly line. On June 6, 1944, hours before D-Day, Roosevelt was given production numbers showing 10,000 B-24s had been built between 1940 and 1944. By the time of Normandy landings, the Liberator was already the most mass-produced plane of any kind in history. "Yes, the Nazis and Fascists have asked for it," Roosevelt said. "And they are going to get it."

President Franklin Roosevelt at the establishment of the first Yugoslavian combat unit in the US Army Air Force with the commissioning of four B-24 Liberator four-engined bombers and their delivery to US-trained Yugoslavian crews, at Bolling Field in Washington, DC, October 6, 1943. (AP Photo)

B-24 crews called their plane the "Flying Box Car," "Spam Can in the Sky," and "Old Agony Wagon." Among those crews were airmen who would become famous after the war, like Jimmy Stewart, Clark Gable, Walter Cronkite, and Andy Rooney. 1912 Olympic gold medalist Jim Thorpe and 1936 Olympic gold medalist Jesse Owens both worked at Ford's Willow Run plant.

B-24 Liberator, set afire by heavy flak over the Weiner Neustadt aircraft plant in Austria during a bombing raid, November 2, 1943, nears a church with a 5-foot-by-13 foot hole in its fuselage by flames. Smoke trails behind. The ship plunged to earth shortly after photo was taken. (AP Photo)

"I was a ball turret gunner on a B-24 Liberator. My first experience in a ball turret was in combat. It was not exhilarating. It was terrifying," said one B-24 crewman. "All just flak. Flak — that's what killed most of our people. In the ball turret, you had a good view. It was hell. You could see the bombs land. We weren't sending enough of the bombs down as far as I was concerned. I didn't care what we did to the people on the ground. They were shooting at me and I wanted them gone."

Of the 416,800 Americans killed in combat during World War II, 79,265 of them were airmen.

B-24 Liberator bombers from the 15th Army Air Force sweep over snow-capped mountains en route to a highly successful air assault on enemy aircraft factories at Wiener Neustadt, south of Vienna, Austria, on April 12, 1944. (AP Photo)

By the end of the war, the Ford company, other manufacturers in Detroit, and the whole of the US war industry had produced a staggering amount of material. The US built 324,750 airplanes during the war, more than Britain and the Soviet Union, the second- and third-biggest producers, combined. Of those planes, 18,482 were Liberators, and 8,685 of them were built at Ford's Willow Run plant.

This B-24 Liberator of the 15th Allied Air Force completed 105 missions throughout Africa, the Mediterranean, northern Europe, and the Balkans. Bomb-loads dropped and German planes bagged complete the imposing score painted on the "Chung-a-lug" by the proud crew members at an airfield in England, May 31, 1944. (AP Photo)

"The number of people involved in making it, in servicing it, and in flying the B-24 outnumbered those involved with any other airplane, in any country, in any time," Stephen E. Ambrose wrote in "The Wild Blue." "It would be an exaggeration to say that the B-24 won the war for the Allies. But don't ask how they could have won the war without it."

Despite its capabilities and performance, the B-24 was already obsolete by the end of the war. The Army had started testing its "superbomber," the B-29 Superfortress, in summer 1943. The B-29 had a better range and could carry a bigger payload than the B-24, and its pressurized cabin let it fly higher. On August 6, 1945, a B-29 dropped the "Little Boy" atomic bomb on Hiroshima. Three days later, another B-29 dropped the "Fat Man" atomic bomb on Nagasaki. Japan surrendered on September 2.

Flames sweep out of the fuselage of a B-24 from the Army 15th Air Force after it was mortally hit by a ME-109 German plane during a mission over the synthetic-oil refineries at Vienna, Austria, July 7, 1944. The bomber broke in two before taking its final plunge. The entire crew was seen to bail out safely. (AP Photo)